1. Field of the Invention
The present invention relates to a protective device for electric control circuits, more particularly to a protective device for an electric control circuit of the type which includes a plurality of electrical loads different in input rating for connection to a commercially available source of electricity.
2. Discussion of the Prior Art
In an electric control circuit for an ice making machine, a single-phase three-wire system (for instance, 60 Hz, 120/240 V) is adapted as a commercially available source of electricity. In a refrigeration system of the ice making machine, a refrigerant compressor is driven by an electric motor the rating voltage of which is 240 volts, a cooling fan or blower for a condensing coil is driven by an electric motor the rating voltage of which is 120 volts, and a solenoid hot gas valve is arranged to be energized by the rating voltage of 120 volts. In the ice making machine, an electric motor of a water pump and a solenoid water valve are arranged to be energized by the rating voltage of 120 volts. Various electronic elements for control of the solenoid valves and motors are arranged to be activated under control of a transformer which is applied with the input rating voltage of 120 volts. To protect the electrical loads such as the motors and solenoid valves, the transformer is connected to the commercially available source of electricity through a fuse which is arranged to be melted when erroneously applied with the input rating voltage of 240 volts.
In the above arrangement, the fuse is useful to protect the electronic elements from damage caused by an error in connection to the commercially available source of electricity. It is, however, necessary to stock some spare fuses for replacement with the melted fuse. In the single-phase three-wire system, it is difficult to quickly discriminate as to whether or not the two input buses for 240 volts have been correctly selected from the three input buses. If the two input bases for 120 volts are connected in an error to the commercially available source of electricity to be applied with the input rating voltage of 240 volts, the fuse will be frequently disconnected. Moreover, the fuse will be disconnected due to aged variation thereof even when it is applied with a normal input rating voltage.